API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

3-94 API RECOMMENDED PRACTICE 581
of ignition. A suitable cloud dispersion model that can handle plumes (continuous release with steady state
analysis) as well as puffs (instantaneous releases that required a transient model) should be used to evaluate
the amount of flammable material that exists in the cloud at the time of ignition.
5.8.5.4 Explosion Yield Factor
An important parameter in the evaluation of the vapor cloud is the explosion yield factor,η . This is an empirical
value that determines how much of the combustion power in the cloud is released into the pressure wave.
Where the flammable mass in the cloud is calculated as the portion of the cloud between the LFL and the UFL
of the flammable material, a conservative value for the explosion yield factor of 1.0 should be used.
Where the flammable mass is based on the total amount of flammable fluid released, then a yield factor, η ,
with a range of between 0.03 ≤η
≤ 0.19 is typically used, and this is a function of the material released. For
example, typical hydrocarbons have a yield factor of 0.03, while highly reactive fluids, such as ethylene oxide,
have yield factors up around 0.19. Additional yield factors are provided by Zebetakis [26] .
5.8.5.5 Determination of Blast Overpressure
a) General—There are several approaches to estimating the overpressure that results from a VCE. The
first method is the TNT equivalency method, explained in Reference [27] and detailed in Section 5.8.5.5
b). In this method, the source of the explosion is assumed to be at a point (point source model) and the
characteristics of the explosion are similar to that of a TNT explosion. This approach will likely result in
conservative estimates of the damage at locations closest to the source of the explosion. The TNT model
has been adopted for its ease of use, ability to be consistently applied, and effectiveness in conservatively
modeling the damage potential of VCEs.
Another model that will not be presented here is more complicated and highly dependent on user
experience and knowledge but can provide more accurate (less conservative) results in the near field of
the explosion. This method is known as the TNO multi-energy method (MEM), and it focuses on the
characteristics of the site, rather than on the size of the release. This method recognizes that portions of
the vapor cloud that are obstructed or partially confined could undergo blast-generating combustion. The
key site characteristics that must be identified and classified by the user are equipment congestion and
flame confinement. Due to lack of reliable guidance in the current research on congestion and
confinement, it is very challenging for the user to consistently apply this approach and, therefore, is not
recommended for RBI purposes where consistency is key.
Yet another model is the Baker-Strehlow-Tang Energy Model [27] , which essentially uses the same TNO
multi-energy methodology, but along with congestion and flame confinement, it includes fuel mixture
reactivity as a key parameter. As with the TNO MEM, the Baker-Strehlow-Tang approach requires user
judgment to classify the site’s congestion and flame confinement, which inherently leads to inconsistent
applications. It is, therefore, not a recommended approach.
b) TNT Equivalency Method—The TNT equivalency method, presented in CCPS [17] , determines the amount
of available energy in the cloud and relates this to an equivalent amount of TNT using Equation (3.169).
W
TNT
η ⋅mass
HC
vce s
= (3.169)
TNT
⋅HC
For mixtures, a mole weighting of the individual component heats of combustions can be used to estimate
the heat of combustion for the mixture in the cloud. The heat of combustion of TNT, HC , is
approximately 4648 J/kg (2000 Btu/lb).
TNT